Fibre-matrix materials, more particularly SMCs and BMCs, are well known to the skilled person and are described in textbooks. (G. Erhard: Konstruieren mit Kunststoffen. Munich: Hanser-Verlag, 1993, ISBN 3-446-17397-8; M. Flemming, G. Ziegmann, S. Roth: Faserverbundbauweisen. Fertigungsverfahren: Fertigungsverfahren mit duroplastischer Matrix. Springer-Verlag, 1998, ISBN 3-540-61659-4).
In many cases the SMCs and BMCs comprise unsaturated polyester resins in styrene as a curable component. In order to reduce contraction in the course of curing, a shrinkage reducer additive or LP additive (low profile additive) is added. This additive is usually a saturated polyester. The materials further comprise fillers, an example being finely ground calcium carbonate. Magnesium oxide is used in order to control the rheology. Reinforcing fibres most often used are glass fibres. Various additives are employed for wetting and incorporating the fillers and the glass fibres. Materials are cured by addition of a peroxide. The BMCs are compression-moulded in complicated moulds. The SMCs are processed to form sheets. This is done in presses at e.g. 140° C.-160° C. in 1 to 5 minutes. The aim of the processing is to produce mouldings which do not comprise emissible chemicals, since in that event the mouldings are accompanied by an odour. The substances emitted are either residual styrene that has not been polymerized, or fragments formed as a result of decomposition of the peroxides.
Benzene is particularly critical in this respect. It is formed in the decomposition of tert-butyl perbenzoate. For this reason, SMCs and BMCs are frequently cured with percarbonates which contain no aromatic structures. One example of such is tert-butyl peroxy-2-ethylhexyl carbonate (Trigonox® 117, Akzo Nobel).
There has been no lack of attempts to eliminate the problems associated with the emission of chemicals.
Accordingly, international patent application WO 03/055946 describes the use of mixtures of peroxides which have their half-lives at 75-95° C. and at 105-125° C. This is said to lead to lower emissions and intrinsic odours on the part of the moulding materials. Nevertheless, it has not been possible by these means to eliminate the problems associated with the use of peroxides.
European patent application EP 1 621 567 describes the use of uretdione diisocyanates. For example, Desmodur TT is coupled by an isocyanate group to the LP additive. The other isocyanate group reacts with the emissions-relevant elimination products from the peroxides. Fundamentally, however, this measure does not alter the fact that emissions-relevant elimination products are initially produced, and then must subsequently be scavenged. It would be desirable, therefore, if such elimination products were not even produced.
DE 26 32 294 describes the preparation of various benzpinacol silyl ethers and the use thereof as initiators for radical polymerization. These initiators act similarly to peroxides, but do not have the hazard potential of the latter. Their activity as radical initiators derives from the homolytic scission of the extremely long tetraphenyl-substituted C—C single bond. In contrast to peroxides, the scission is endothermic and, in the absence of a monomer as reactant, is also reversible. However, these benzpinacol silyl ethers can be employed only in solutions. Using them in SMCs and BMCs, therefore, would result in emissions of volatile solvents.
Benzpinacol itself can also be used as a radical initiator for the polymerization of unsaturated polyester resins (Heinrich Wolfers et al. in Die Makromolekulare Chemie, 180, 3, pp. 649-655). Benzpinacol, however, is insoluble in styrene and styrenic unsaturated polyester resins. The use of benzpinacol as a radical initiator is described in DE 10 2009 026 185.
Also known, from European patent application EP 0 033 750, are radically polymerizable dental compositions which comprise benzpinacol as initiator. The cured dental compositions, however, have large, leaf-like blisters. Only 2,2′-dialkylbenzpinacols do not exhibit this disadvantage.
A disadvantage of customary SMCs/BMCs is that on account of the decomposition compounds emitted from the initiators, they have an unpleasant odour, thereby restricting their possibilities for use.